The product of the recA gene of E. coli is a key transmitter of the effect of carcinogens in the bacterium. When activated, the recA protease mediates induction of a diverse set of genes called "SOS functions". The protease cleaves certain bacteriophage repressors and the product of the lexA gene. The latter controls (represses) a diverse set of genes. We understand in considerable molecular detail the action of a bacteriophage repressor, and this has clarified our picture of how lysogens are induced by carcinogens. We seek to extend our understanding of how recA interacts with lexA protein and the phage repressors, and we wish to further our understanding of how lexA protein functions. We shall isolate mutants bearing changes in and around the recA gene that affect proteolytic function or its control by lexA. Strains that produce large amounts of lexA protein will be constructed, thereby allowing analysis of the protein and its function in vitro. Cleavage of lexA protein and of various bacteriophage repressors by wild-type and mutant recA proteins will be studied in vitro. The interaction of lexA with controlling regions of a variety of genes including umuC will be analyzed in vitro and in vivo. The umuC gene, which codes for a function required for UV damage-induced mutagenesis, will be isolated. Strains producing large amounts of this gene product will be constructed and the protein studied in vitro.